1. Field of the Invention
The present invention relates to a power converter, and more specifically relates to the control of a switching mode power converter.
2. Description of the Related Art
Power converters have been used to convert an unregulated power source to a regulated voltage or current. FIG. 1 shows a traditional power converter. A control circuit 10 generates a switching signal VSW for switching a transformer 30 via a transistor 20. A resistor 40 is applied to sense a switching current IP of the transformer 30 for switching control. A resistor 45 is equipped to determine a switching frequency of the control circuit 10. A terminal FB of the control circuit 10 is connected to an output of a feedback circuit 50. The feedback circuit 50 is further coupled to an output of the power converter for generating a feedback signal VFB. According to the feedback signal VFB, the duty cycle of the switching signal VSW determines the power delivered from an input of a power source to an output of the power converter. Although the switching technology reduces the size of the power converter, switching devices generate electric and magnetic interference (EMI) that interferes the power source and environment. The EMI solutions, such as the EMI filter, the transformer shielding etc., are thus required to equip in the power converter for reducing the EMI. However, such EMI solution inevitably causes power consumption and increases the cost and the size of the power converter. In recent development, many prior arts have been proposed to reduce the EMI by using frequency modulation or frequency hopping. For example, “Reduction of Power Supply EMI Emission by Switching Frequency Modulation” by Feng Lin and Dan Y. Chen, IEEE Transactions on Power Electronics, VOL. 9. No. 1. January 1994. “Effects of Switching Frequency Modulation on EMI Performance of a Converter Using Spread Spectrum Approach” by M. Rahkala, T. Suntio, K. Kalliomaki, APEC 2002 (Applied Power Electronics Conference and Exposition, 2002), 17-Annual, IEEE, Volume 1, 10-14, March, 2002, Pages: 93-99. “Offline Converter with Integrated Softstart and Frequency Jitter” by Balu Balakirshnan, Alex Djenguerian, U.S. Pat. No. 6,229,366, May 8, 2001; and “Frequency Jittering Control for Varying the Switching Frequency of a Power Supply” by Balu Balakirshnan, Alex Djenguerian, U.S. Pat. No. 6,249,876, Jun. 19, 2001. However, the disadvantage of the prior art is that frequency modulation generates undesirable ripple signal at the output of the power converter. The undesirable ripple signal generated by frequency modulation could be realized by the following description. An output power PO of the power converter is the product of an output voltage VO and an output current IO of the power converter, which is given by,PO=VO×IO=η×PIN  (1)
An input power PIN of the transformer 30 and the switching current IP can be respectively expressed by,
            P      IN        =                  1                  2          ×          T                    ×              L        P            ×              I        P        2                        I      P        =                            V          IN                          L          P                    ×              T        ON            Where η is the efficiency of the transformer 30; VIN is an input voltage of the transformer 30; LP is a primary inductance of the transformer 30; T is a switching period of the switching signal VSW; TON is an on-time of the switching signal VSW. The equation (1) can be rewritten as,
                              P          O                =                  η          ×                                                    V                IN                2                            ×                              T                ON                2                                                    2              ×                              L                P                            ×              T                                                          (        2        )            
The switching period T varies in response to frequency modulation. As shown in equation (2), the output power PO will vary as the switching period T varies. Therefore, an undesirable ripple signal will be generated as the output power PO varies.
Another disadvantage of prior art is an unpredictable range of the frequency modulation, which is correlated to the setting of the switching frequency. This would decrease the effect of the EMI reduction when the switching frequency is programmed in response to various application needs.
An object of the present invention is to provide a method and circuit of switching frequency modulation to reduce the EMI for a power converter. Different from prior arts, the present invention will not generate the undesired ripple signal at the output of the power converter. Another object of the present invention is to develop a frequency modulation unrelated to the setting of the switching frequency of the power converter.